In situ leaching method for determination of chloride in concrete pore water

Abstract

In the in situ leaching (ISL) method, pore water ionic content is determined in small cavities drilled in mortar/concrete specimens. Prior investigations have demonstrated the ISL applicability to obtain pH and nitrite ion concentration in concrete/mortar pore water. The application of the method is extended here to determine pore water chloride ion concentration (and pH) within practical test times in mortars and concretes prepared in the laboratory and in concrete cores extracted from a bridge deck in deicing salt service. Spatial resolution for the determination of composition profiles is also illustrated. Modeling of the ISL process indicates that chloride binding accelerates the approach toward a terminal cavity concentration, reducing test duration to practical lengths for moderate permeability concretes. This acceleration can be attributed to maintaining a higher gradient of free chloride near the cavity wall due to the release, during leaching, of previously bound chloride. Consequently, there is a faster chloride buildup in the cavity water compared with the no-binding case. Experimental chloride and pH results obtained by the ISL test in mortar samples show good agreement with those from the pore water expression (PWE) method. Also, examples are presented of application of ISL data to obtain chloride binding isotherms, and pore water chloride to hydroxide ratio relevant to assessing conditions for corrosion of steel reinforcement. The ISL method presents a less costly and less disruptive alternative to PWE for pore water analysis. It is noted, however, that in a few instances ISL could not be implemented because of excessive absorption of cavity water by the surrounding medium.